Cartridge filters key parameter is surface area, enabling them to operate for a period of time. As water flows through the filter, the material is able to catch unwanted contaminants in the water. Properly sized cartridges require minimal maintenance, and may only need to be rinsed off or replaced a few times per year. Cartridge filters can be of many different forms. There are carbon block, carbon granular activated, sediment, ion exchange, ultra filtration, submicron, and absolute filter cartridges. The second step in the application is sizing the cartridge for efficacy and reasonable filter change periods. Lastly the designer needs to consider how many filters are needed and if they are plumbed in parallel or in a series.